Protection of active implantable medical devices is particularly important to provide normal functioning of the implantable device, regardless of the outside environmental conditions, and particularly in the presence of externally originated electromagnetic interference. The various electrodes connected to the implantable device can sense (couple) all sorts of electromagnetic radiation emanating from various external sources. These external sources include engines, televisions, induction plates, portable phones, door locks using R-F tag "keys", anti-theft protection systems, etc. A certain number of medical devices used in the course of surgical interventions also generate electromagnetic radiation, such as electrical R-F surgery devices, cauterization instruments with alternating current, external defibrillators, etc.
Such externally originated electromagnetic radiation can induce or produce parasitic signals in the active implantable medical device which perturb the operation of the device, which perturbations can be very different in nature from the normal signals: prolonged or brief overvoltages (i.e., an excessive signal amplitude), induced currents, radio or high frequency signals of a wide variety of spectral characteristics (in particular, the spectrum spreading typically from one to several megahertz), etc. The different natures of these perturbations render it difficult to provide protection for the pacemaker that is complete and efficient in all circumstances of electromagnetic perturbations.
The externally originated parasitic signals appearing in the active implantable medical device, which are often designated under the term "EMI" (ElectroMagnetic Interference) and hereinafter called "ElectroMagnetic perturbations", or more simply "perturbations", have, for a first effect, to superpose a perturbation on the cardiac signal. This results in a risk of disturbing the functioning of the pacemaker, which would respond to a perturbation treating it as a condition that is in the accepted range of possible conditions, but which does not reflect the actual condition (e.g., treating a perturbation as a natural (sensed) or paced (stimulated) cardiac event). Therefore, the pacemaker should detect perturbations and, if they exceed a given threshold in spite of the protection circuits provided or of circuits inside the pacemaker to suppress perturbations that are coupled into the pacemaker, then the pacemaker should switch into a "perturbation" operating mode, where it will function in an autonomous manner until the level of the coupled perturbation falls under a predetermined threshold.
Another adverse effect of perturbations sensed by the pacemaker is the risk of destruction or damage due to voltages or excessive currents introduced in the pacemaker circuits; it is therefore absolutely indispensable to limit voltages and currents in cardiac probes at the input of the device.
In this regard, very strict standards have been defined in the industry. For example, the standard designated CENELEC EN 50061 Amendment 1 "Security of implantable cardiac pacemakers", defines the levels of required minimal protection as well as a number of test procedures to verify the conformity of pacemakers to the standard.
Up until now, the protection against perturbations of external origin has been realized by the use of Zener diodes, in the form of discrete components mounted on the microchip (i.e., integrated circuit or circuits) of the pacemaker at the input connector where the various probes (leads) are connected to the device. These Zener diodes insure a limit of the overvoltages susceptible to appear at the input of the pacemaker (the voltage limit is the voltage of the Zener diode). The diodes are provided to insure a symmetrical protection so as to avoid in particular the effect of demodulating high frequency perturbations.
A problem with the known protection provided by Zener diodes placed at the input of the pacemaker leads is the disadvantage of an increase in the congestion of the pacemaker circuits, due to the need to provide a relatively high number of such supplementary discrete components (one Zener diode at each in/out connection of the pacemaker, necessitating at least two diodes per lead). This is contrary to the general research goal of increased miniaturization of implantable medical devices, particularly in complex devices such as multiple chamber pacemakers, pacemakers connected to a sensor of enslavement (i.e., a rate response pacemaker that monitors a physiological parameter indicative of cardiac output requirements from which a rate responsive pacing rate can be derived), defibrillators, etc.
Concerning efficiency, protection circuits using Zener diodes insure a satisfactory protection against the high voltages linked, for example, to defibrillation shocks. On the other hand, however, perturbating voltages of lower amplitude can be sensed or demodulated in case of ElectroMagnetic Interference and can cause problems with the low voltage circuits of the pacemaker. Classic Zener diode protection circuits are not as effective against such perturbations. Hence, there is a continuing need for improved protection in this area.
The need for improved protection is particularly apparent in the case where circuits of the pacemaker comprise integrated active components functioning with relatively low nominal control voltages, e.g., the gate voltage of an MOS transistor (MOSFET). Up until now, these components using such low control voltages have not been regularly used in implantable devices, in part because the risk of malfunction due to external perturbations has been so high. This has resulted in depriving the field of the advantages of the most recent semiconductor technologies, particularly the benefits of their very high density of integration and their low power consumption--characteristics which are nevertheless particularly desirable for autonomous implantable devices.